(1) Field of the Invention
The present invention relates to fixing devices provided in image formation apparatuses such as printers and copiers, and relates in particular to technology of detecting abnormal heat generated in a fixing device that utilizes a resistance heating layer as a heating element.
(2) Related Art
In recent years, fixing devices that utilize a resistance heating layer as a heating element have been commonly used as fixing devices for image formation apparatuses such as printers and copiers. The resistance heating layer generates Joule heat when supplied with power. In such a fixing device, the resistance heating layer is directly supplied with power to generate heat. Therefore, such a structure improves the heat usage efficiency and reduces the time period required for warming up.
The resistance heating layer is formed by dispersing conductive material, such as metal, in a body made of insulative material, such as heat-resistant resin. In addition, the resistance heating layer is usually coated with an insulative layer, since direct contact with the layer may cause electric shock. For example, Japanese Patent Application Publication No. 2009-109997 discloses a fixing device utilizing a resistance heating layer coated with an insulative layer, which serves as a heating element.
Meanwhile, since the insulative layer is very thin (e.g. on the order of several hundred micrometers), the insulative layer may be damaged due to entrance of foreign objects or contact with recording sheets. If the damage reaches the resistance heating layer, particularly in the event of the occurrence of a flaw in a non-parallel direction to the current flow (in particular, the perpendicular direction to the current flow), the current will detour around the flaw and be concentrated at the edges of the flaw. As a result, the current density locally increases in areas around the edges of the flaw. This leads to abnormal heat generated in the areas with the locally-increased high current density in the resistance heating layer.
Such abnormal heat, if left without any treatment, may damage the fixing device, and can be a cause of overheating of the fixing device or catching fire. Therefore, it is necessary to detect the occurrence of abnormal heat without overlooking it, and when the abnormal heat is detected, it is necessary to prevent further damage to the fixing device by taking a necessary measure such as cutting off the power supply to the fixing device.
A fixing device is provided with temperature detector elements such as a thermistor and a thermostat. However, the detection range of such temperature detector elements is usually narrow. Therefore, there is a problem that the occurrence of abnormal heat in the resistance heating layer is overlooked, depending on the location of the abnormal heat.
Several approaches can be conceived of as solutions to this problem. For example, the number of temperature detector elements may be increased to cover the entire surface of the resistance heating layer. It is also possible to configure the temperature detector elements to be movable, and detect the temperature of the entire surface of the resistance heating layer by moving the temperature detector elements.
These approaches expand the range of the abnormal heat detection by the temperature detector elements, and reduce the frequency of overlooking the occurrence of abnormal heat.
However, the first approach above has a following problem. That is, since the number of temperature detector elements that can be provided is limited in light of reduction of the manufacturing cost, it is difficult to sufficiently expand the detection range of the abnormal heat, and it is therefore difficult to sufficiently increase the detection sensitivity of the abnormal heat.
The second approach is also problematic, because it takes a long time to move the temperature detector elements across the entire surface of the resistance heating layer, and accordingly the period of the detection cycle at each detection point will be long. That is, when abnormal heat occurs, it takes a long time before detecting it. It is therefore impossible to promptly take a necessary measure when abnormal heat occurs, and the damage to the fixing device could progress during this delay.